The present invention provides a membrane assisted evaporation process (MAE) for economically and reliably removing water added to brine. The process comprises using low-grade waste heat and air to evaporate water from diluted salt brines when the water moves across a membrane in a liquid state.
Salt caverns have been used for storage of oil, particularly crude oil. When oil is to be pumped out of a salt cavern, a brine solution is pumped in to replace the oil. The brine concentrations are preferably within the range of 14 to 22.5% (by weight) of salt (mostly NaCl). However, the brine is stored in ponds and the ponds can take in rain water that results in a net dilution of the brine with pure water. The effect of diluted brine is a slow destruction of the salt caverns through removal of salt from the walls and eventual collapse of the caverns. Therefore, there is a need in the art to remove water from holding salt ponds that gets into such ponds from rain water and concentrate the brine to near saturation.
For example, salt caverns adjacent to a refinery in Ontario, Canada and in Texas each annually takes in about 150,000 barrels (38 gallons) of rainwater per year on average. This refinery can either develop a process to remove water from the brine ponds to near saturation or build indoor pool holding tanks to prevent rainwater dilution. Previous attempts at brine regeneration have attempted to use ultrafiltration membranes and specifically hydrophobic membranes (e.g., polysulfone) in a pervaporation or membrane distillation-type process. The pervaporation or membrane distillation process uses a hydrophobic membrane and heat to drive water in the vapor phase from the heated brine side to the permeate side. Water is vaporized in the heated brine and migrates across the membrane still in the gas phase to the permeate side driven by a vacuum. Generally, salt is rejected by the membranes and as a result, crystals form on the brine side and foul the membrane. In another approach (U.S. Pat. No. 4,316,774) the air is heated on the permeate side but this is wasted because air (unlike a liquid) cannot hold much heat. Significant problems with membrane fouling have been encountered and the membrane wetting and fouling prevented vapor from permeating into the membrane, effectively shutting down the process.
The need of concentrating diluted brine solutions arises in petroleum storage facilities in st caverns as indicated above. However, similar processes are needed for brine recovery for the chloralkali industry and for treating cooling tower blowdown waste water.
Therefore, there is a need to develop concentration techniques using membranes that can further concentrate brine to near saturation without local crystallization on the membrane from the high salt concentrations and without utilization of significant amounts of energy in an evaporation process at such high boiling points (due to the high salt concentration). The needed membrane should also be able to resist hydrolysis caused by heat, high pH and cleaning solutions as it will operate in harsh and caustic conditions. The inventive membrane and processes address such needs.
The present invention provides an osmotic membrane evaporation process (OME) for removing water from brine to further concentrate the brine to near saturation. The process for concentrating diluted feed of brine or other aqueous solution for concentration comprising:
(a) providing a hydrophilic membrane having a rejection property of 500 kDa cutoff or lower having a first side designed to be in contact with diluted feed, and having a second side designed to be in contact with air, wherein the hydrophilic membrane is not able to reject salt;
(b) pumping the diluted feed at a temperature of from about 10xc2x0 C. to about 100xc2x0 C. across the first side of the hydrophilic membrane while blowing an air stream or other gas across the second side of the hydrophilic membrane; and
(c) removing water from the diluted feed by evaporating the water into the air stream blown across the second side of the hydrophilic membrane.
Preferably, the hydrophilic membrane is an asymmetric hydrophilic membrane further comprising a fabric layer on the second side of the membrane to provide mechanical strength for the membrane. Most preferably, the fabric is polyester net, having about 60% open area and about 0.07 mm thick. Preferably, the fabric is a silkscreen material. Preferably, the hydrophilic membrane is made from a cellulose material or polyvinyl alcohol. Most preferably, the cellulose material is selected from the group consisting of cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose acetate butyrate, cellulose acetate, proprionate, and combinations thereof.
Preferably, the diluted feed is heated using any available heat source to a temperature of from about 10xc2x0 C. to about 95xc2x0 C. Most preferably, the diluted feed is heated to a temperature from about 50xc2x0 C. to about 95xc2x0 C. Preferably, the air stream on the second side of the membrane is blown at a velocity of from about 5 cm/sec to about 100 m/sec. Most preferably, the velocity of air across a membrane is about 100 cm/sec.
The present invention further provides a device for osmotic membrane evaporation and brine or other aqueous concentration (feed), comprising:
(a) a hydrophilic membrane having a first side and a second side, having a rejection property of 500 kDa cutoff or lower, wherein the hydrophilic membrane is not able to reject salt;
(b) an enclosed feed flow chamber having an inlet and an outlet and defined by communication with the first side of a hydrophilic membrane;
(c) an enclosed air flow chamber having an air flow blowing means adjacent to an inlet and an outlet and defined by the second side of the hydrophilic membrane; and
(d) a heating means for the feed located adjacent to the inflow of the brine flow chamber.
Preferably, the hydrophilic membrane is an asymmetric hydrophilic membrane further comprising a fabric layer on the second side of the membrane to provide mechanical strength for the membrane. Most preferably, the fabric is polyester net, having about 60% open area and about 0.07 mm thick. Preferably, the fabric is a silkscreen material. Preferably, the hydrophilic membrane is made from a cellulose material or polyvinyl alcohol. Most preferably, the cellulose material is selected from the group consisting of cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose acetate butyrate, cellulose acetate proprionate, and combinations thereof.